1. Field of the Invention
The invention relates to a sharable filter in a transceiver, and more particularly, to a flexible filter structure switching between different passbands on demand.
2. Description of the Related Art
Filter design plays a very important role in communication systems. An ideal receiver channel filter can minimize signal distortion by maximizing signal to noise ratio, rejecting out-of-band interference and limiting the noise bandwidth to a satisfactory level. The goal of less distortion, however, conflicts with the design goal of providing more rejection of noise and interference. Thus, receiver filters are always burdened with undesirable trade-offs.
FIG. 1 shows a conventional direct conversion transceiver 100. When in receiver mode, inbound RF signal RFIN is received and amplified by a low noise amplifier (LNA) 102, and a first mixer 104I and a second mixer 104Q individually render mixer results #Iin and #Qin that are then filtered in the low pass filters (LPF) 106I and 106Q. Thereafter, filter results RXI and RXQ are output for further procedures in an analog to digital converter (ADC) unit (not shown). Conversely, when in transmission mode, outbound baseband signals TXI and TXQ are individually provided to the LPF 110I and 110Q. Filter results #IOUT and #QOUT are then mixed in the first up converter 112I and second up converter 112Q, and a combination result is sent to the variable gain amplifier (VGA) 114 for gain adjustment, and the power amplifier 116 transmits the outbound RF signal RFOUT after performing power amplification.
Among the LPFs 106I, 106Q, 110I and 110Q, the most important parameter is filter bandwidth. In a typical communication system, the transmitter LPFs 110I and 110Q are designed to achieve wider bandwidth than that of the receiver LPFs 106I and 106Q. However, LPFs of different bandwidth parameters are redundantly implemented in one transceiver, causing significant waste of chip area and costs. As known, filters consume significant chip area and power, which is undesirable for most recent 3C applications of which compactness and power efficiency are basic requirements. Therefore, a new filter architecture to eliminate redundancies of components in a transceiver is desirable.